Client computing devices may be used to request content pages and other network-accessible content from content servers. Content pages can include text, images, videos, animations, and the like. In a typical application, a client device transmits a request for a content page to a content server. The request can be formatted and communicated according to standard protocols understood by both the client computing device and the content server, such as the Hypertext Transfer Protocol (“HTTP”). In response to some requests, the content server can prepare a response in the form of a Hypertext Markup Language (“HTML”) document. HTML documents may have embedded references to separate files and objects, such as images, executable code files, and the like. An HTML document may be transmitted to the client computing device in response to an HTTP request. The client computing device can process the HTML document, identify embedded references to separate files and objects, and request those files and objects, e.g., by issuing HTTP requests to the same server or to a different server.
Conventionally, content pages, such as those defined by HTML documents, may be tested and debugged by executing a debugging application on the same computing device that is rendering or executing content to be debugged. For example, a debugging application may launch a browser application to render an HTML file and execute any embedded reference. The debugging application may have access to the state of the browser application during rendering and execution, and can inspect element values, execute instructions, and perform other debugging operations. In some cases, a separate computing device may access the state of a browser application during rendering and execution. For example, a first client computing device may execute a debugging application, and a separate client computing device, connected to the first client computing device via a universal serial bus (“USB”) cable or some other direct communication link, may execute a browser application that renders an HTML file. The separate client computing device may be configured to allow the debugging application, running on the first client computing device, to perform debugging operations (read/write program state, execute commands, etc.)